Method and apparatus for controlling frequency of a multi-channel transmitter

ABSTRACT

A method an apparatus for controlling frequency of a multi-channel transmitter. A reference frequency is generated and emitted to a thermally stable frequency discriminator. The thermally stable frequency discriminator emits a dc voltage to a plurality of individual oscillators. The individual oscillators in continuous wave mode or modulated by baseband signals are combined into a multi-channel signal at an intermediate range of frequencies. The multi-channel intermediate range of frequencies are preferably amplified and emitted as an output signal. At relatively lower frequencies, such as 4-6 GHz, up-conversion of the output signal is not necessary. At relatively higher frequencies, an up-converter can be used to employ a second thermally stable frequency discriminator.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method and apparatus for controllingfrequency of a thermally stable microwave oscillator bank.

2. Description of Prior Art

U.S. Pat. No. 4,629,999 teaches a phase-locked loop capable ofgenerating stable frequency signals. Voltage controlled oscillators aresequentially coupled to an output of a phase sensitive detector by amultiplexer. Oscillator outputs are sequentially coupled in synchronismwith a multiplexer, to a programmable divider which is coupled to aninput of the phase sensitive detector. Another input is coupled to areference oscillator.

U.S. Pat. No. 4,694,260 discloses a microwave frequency discriminatorfor transforming a frequency modulated signal into a low frequencydemodulated signal, wherein the discriminator is used with microwaves.An oscillating circuit has a frequency controlled by an externallyapplied dc voltage which is applied to a controlled circuit, such as acircuit including varactor diodes. The '260 patent discloses neithermultiple microwave oscillators nor thermally coupling or frequencytracking abilities of such microwave oscillators.

U.S. Pat. No. 3,614,640 discloses a frequency discriminator whichoperates without inductive components. A signal is divided into twochannels. One channel acts as an active filter which connects to apositive detector having an output which is transmitted to a mixingdevice. The other channel acts as an active filter which connects to aminus detector having an output which is also transmitted to the mixingdevice. Each active filter uses no inductors but has a transfer functionequivalent to that of a tuned circuit.

U.S. Pat. No. 3,868,606 discloses a highly stable HF and VHF source thatuses a Q-multiplied quartz crystal resonator which includes a frequencydiscriminator section of an AFC stabilization loop, to provide alow-noise spectrally pure frequency source.

U.S. Pat. No. 4,333,062 teaches a temperature-stabilized MIC solid-stateoscillator which has two strip line resonators with chip capacitorspositioned serially in the middle of the strip lines. The chipcapacitors have linear capacitance temperature characteristics.

U.S. Pat. No. 4,345,221 teaches a signal generator with a digitaltemperature compensation circuit that has high and low frequency quartzoscillators. The low frequency oscillator is used for temperaturecompensation.

U.S. Pat. No. 3,787,612 discloses signal processing circuits for atelephone receiver which include acoustic surface wave devices fortuning and video demodulation. Surface wave discriminators have similarthermal characteristics and thermal exposure which cause temperaturetracking of the frequency characteristics and thus temperaturestabilizing of the receiver.

It is apparent from the teachings of the known prior art references thatthere is a need for a frequency controller that uses a plurality ofelectrically and thermally matched oscillators, as well as a referenceoscillator and a thermally stable frequency discriminator, to producemodulated individual signals that can be combined into an intermediatefrequency output signal.

SUMMARY OF THE INVENTION

One object of this invention is to provide a thermally stable microwaveoscillator bank which employs one stable frequency source and whichgenerates a reference frequency, and also employs one or more thermallystabilized frequency discriminator, depending upon whether up-conversionof the output frequency is necessary.

Another object of this invention is to provide a microwave oscillatorbank which is capable of generating an array of millimeter-wavefrequencies through direct modulation of the oscillators within theoscillator bank.

Still another object of this invention is to provide a microwaveoscillator bank and a corresponding frequency controller that maintainstemperature stability within the oscillator bank, throughout arelatively long-term useful life of the components.

The above and other objects of this invention are accomplished with amethod for controlling frequency of a multi-channel transmitter in whicha reference frequency is generated and emitted to a thermally stablefrequency discriminator. Such thermally stable frequency discriminatoremits a dc voltage to a plurality of individual oscillators. Modulationis fed through each individual oscillator to an individual signal andall modulated individual signals are then combined into a multi-channelintermediate frequency. The multi-channel intermediate frequency ispreferably amplified and then emitted as an output signal. However, themulti-channel intermediate frequency can also be directly fed as anoutput signal.

In one preferred embodiment according to this invention, the outputsignal is mixed with a control signal which is emitted at a controlfrequency from a local oscillator. The mixed signal is then preferablybut not necessarily amplified. In one preferred embodiment of thisinvention, the control signal is mixed with a frequency reference signalgenerated from a stable frequency source to form a second mixed signal.The second mixed signal is emitted to another thermally stable frequencydiscriminator which emits a second dc voltage to the local oscillator.The control signal can be taken from a directional coupler which isexposed to the control signal. The second mixed signal can also beamplified.

The modulated individual signals are preferably each at an intermediatefrequency which is less than an output frequency of the output signal.The individual signals are preferably spaced by either one fixedfrequency interval or an integer multiple of a fixed frequency interval.

Frequency control is important in many different applications ofmicrowave and millimeter-wave systems, such as wideband multi-channelcommunications systems and instrumentation, which require frequencytracking among the output spectral components. In many millimeter-wavecommunications systems and instrumentation, oscillator banks mustgenerate highly stable frequencies, on both long-term and short-termbases. Such oscillator banks can operate in a carrier wave (CW) mode orcan be modulated with various baseband signals.

The method and apparatus of this invention provides a highly stableoscillator bank which does not require phase-locked loops, such as insystems that operate at AM, FM, FSK, PAM, PPM and the like modulationtechniques. In systems that require phase-locking, such as those thatoperate at BPSK, QPSK and the like modulation techniques, the method andapparatus according to this invention provides effective control of thefrequencies throughout elements in the oscillator bank, which thusreduces the required capture range of the phase-locked loops in thesystem.

Regardless of the number of individual oscillators in the oscillatorbank, the method and apparatus according to this invention requires onlyone stable frequency reference and one or more thermally stablefrequency discriminators, all of which preferably operate at aconvenient intermediate frequency which is substantially lower than themillimeter-wave band of frequencies. If an oscillator bank systemaccording to this invention operates at relatively low frequencies, suchas 4-6 GHz, then exposing the output frequency to an up-conversioncircuit is unnecessary and only one thermally stable frequencydiscriminator is necessary.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features of this invention will becomemore apparent, and the invention itself will be better understood byreference to the following description of specific embodiments of theinvention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a block diagram of a multi-channel millimeter-wavetransmitter, according to one preferred embodiment of this invention;

FIG. 2 is a block diagram of a multi-channel millimeter-wavetransmitter, according to another preferred embodiment of thisinvention;

FIG. 3 is a block diagram of a multi-channel microwave transmitter,according to still another preferred embodiment of this invention; and

FIG. 4 is a block diagram of a frequency discriminator, according to onepreferred embodiment of this invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a block diagram of a millimeter-wave transmitter, accordingto one preferred embodiment of this invention. Because of relativelyhigher losses in cables and waveguides at millimeter-wave frequencies,signals are preferably transmitted within a band of intermediatefrequencies (IF) to a location near an antenna or another instrument,and are then up-converted to a millimeter-wave band for amplificationand transmission.

As shown in FIG. 1, individual signals 15 are fed to modulator andcombiner 20 which emits a multi-channel modulated signal 21 preferablyat an IF synthesized band, such as below approximately 10 GHz. In onepreferred embodiment according to this invention, a fixed frequencyinterval or a positive integer multiple of a fixed frequency interval ismaintained between the individual modulated signals 21 emitted frommodulator and combiner 20, so that no interference exists between thechannels.

The multi-channel IF signal is preferably sent to a transmitter, forexample by way of a cable or waveguide. Within the transmitter,up-converter and filter 23 translates modulated signal 21 to anappropriate millimeter-wave band preferably for further amplificationthrough power amplifier 24 and transmission through antenna 25.Frequency controller 22 is preferably used to control a frequency of alocal oscillator, such as local oscillator 49 shown in FIG. 2, withinup-converter and filter 23. As shown in FIG. 1, frequency controller 19is preferably used to control the frequency emitted from each individualoscillator within an oscillator bank.

When operating at an outdoor or remote location, the oscillatorpositioned within up-converter and filter 23 can be subjected totemperature variations. The frequency of such oscillator can bemonitored and controlled in order to regulate the emitted frequencyrange.

Referring to FIG. 2, an oscillator bank according to one preferredembodiment of this invention comprises oscillators 30-33. It is apparentthat any number of a plurality of oscillators 30-33 can be employed,according to this invention. Oscillator 33 is intended to represent thenth oscillator within oscillator bank 29. According to one preferredembodiment of this invention, individual oscillators 30-33 which operateat the IF band are thermally coupled with respect to each other. Suchthermal coupling can be accomplished, for example, by positioning allindividual oscillators 30-33 in an enclosed cabinet. Such enclosedcabinets are often used in an indoor environment and are well known bythose skilled in the art of constructing enclosures for oscillatorbanks.

As shown in FIG. 2, reference oscillator 26 generates a referencefrequency f_(r) which is emitted to thermally stable frequencydiscriminator 27. Reference oscillator 26 is preferably positionedwithin the same environment, such as the physical cabinet enclosure, asindividual oscillators 30-33. Also, reference oscillator 26 ispreferably an electrical equivalent to any one of individual oscillators30-33. In preferred embodiments according to this invention, referenceoscillator 26 does not contribute to the IF signal. Reference oscillator26 and oscillators 30-33 are preferably constructed with identicalcircuit design, layout and physical housings. Such similarity betweenreference oscillator 26 and each of oscillators 30-33 result in all ofthe oscillators being electrically equivalent with respect to eachother. Such electrical equivalence can be accomplished by matching thethermoelectric properties of the related semiconductor devices, such asin a manner known to those skilled in the art of constructingsemiconductor devices. For example, the thermoelectric properties ofreference oscillator 26 and oscillators 30-33 can be matched throughwell known measurement techniques apparent to those skilled in the art.

Any undesired offset in frequency between individual oscillators 30-33can be adjusted by mechanical tuning. For example, in a dielectricresonator oscillator, mechanical tuning can be accomplished by adjustinga metal plate above the dielectric resonator. Temperature coefficientsof the resonant frequencies of oscillators with identical design arevery close to each other. For oscillators operating within an indoorenvironment, such as at temperatures between approximately 15° C. and30° C., temperature compensation can be used to minimize any frequencydrift. Frequency drift is detected at the output of reference oscillator26. The frequency f_(r) of reference oscillator 26 is preferably chosenso that as shown in FIG. 2, f₁ <f_(r) <f_(n).

Any change in output frequency causes a dc output voltage from frequencydiscriminator 27. Such dc output voltage can be fed to individualoscillators 30-33 to cancel the shift in frequency between suchoscillators 30-33. Because oscillators 30-33 are preferably temperaturematched with respect to each other, the same dc output voltage can beused to correct the frequency drift between oscillators 30-33.

If relatively small deviations exist between the temperaturecoefficients associated with oscillators 30-33, simple resistivenetworks can be used to alleviate such relatively small deviations, bydistributing the automatic frequency control (AFC) voltage sent toindividual oscillators 30-33. By calibrating the resistive network at ahot end and at a cold end of a temperature range, proper tracking willexist throughout the intended operating temperature range.

If the method and apparatus according to this invention are practiced inan outdoor environment, it is likely that considerable variation inoutput frequency, commonly referred to as free-running, will occur.Other components with an up-converter in filter 23 can be used to changethe frequency of the output signal. As shown in FIG. 2, local oscillator49 can be used to emit a signal to mixer 41 for mixing with themodulated signal 21 from modulator and combiner 20. As shown in FIG. 2,directional coupler 43 takes a sample signal from the output of localoscillator 49 and mixes the sample signal with an output signal fromstable oscillator 47, which is preferably located in an indoorenvironment. The frequency of stable oscillator 47 is preferably chosenat an IF band so that the frequency signal can be sent to up-converterand filter 23 without significant losses. If the distance betweenup-converter and filter 23 and stable oscillator 47 is relatively large,it is apparent that the signal strength at stable oscillator 47 can beincreased prior to mixing at mixer 45. An appropriate component of themixed signal emitted from mixer 45 can be chosen to represent thefrequency condition of stable oscillator 47. Such frequency component,shown in FIG. 2 as f_(LO) -kf_(I), where k is an integer, will reflectany shift in f_(LO). Thermally stable frequency discriminator 46preferably generates a dc correction voltage which is proportional tothe frequency drift in f_(LO), and sends such dc correction voltage asan input to local oscillator 49.

Thus, according to the method and apparatus of this invention, it isnecessary to employ only one stable frequency source, stable oscillator47, at a convenient IF frequency, and one or more thermally stablefrequency discriminators, 27 or 46. Such components are preferablylocated in an indoor environment and thus can be easily temperatureregulated.

FIG. 4 shows a block diagram for a frequency discriminator, such asfrequency discriminator 27 or 46, according to one preferred embodimentof this invention. As shown in FIG. 4, only filter or dispersive network51 and detector diodes 52 are thermally stabilized. The remainingcomponents, such as low-pass filter 53 and dc amplifier 54, can bethermally unregulated.

FIG. 3 shows another preferred embodiment according to this invention ofan apparatus for controlling frequency of a multi-channel microwavetransmitter. In such preferred embodiment, oscillator bank 29 operatesin a relatively lower frequency range where losses due to cables andwaveguides are relatively low. FIG. 3 shows a direct synthesis systemwhich is capable of generating an array of microwave channels up toapproximately 20 GHz. The system shown in FIG. 3 requires noup-conversion function and requires only one thermally stable frequencydiscriminator 27.

In phase-coherent communications systems or certain instrumentation,oscillators should be phase-locked. The method and apparatus accordingto this invention can provide an effective means for preventingoscillators 30-33 from drifting out of a capture range of thephase-locked loops, as a result of temperature change and aging of thecomponents. Each oscillator 30-33 can be controlled by both thefrequency control voltages emitted from frequency discriminator 27, aswell as by controlling an amplified mixer output of a phase-locked loop,which may derive a reference signal from a designated reference. Suchphase control signal should have a much faster response time than theAFC signal emitted from frequency discriminator 27, so that theshort-term stability of reference oscillator 26 as well as its phase canbe easily regulated. In one preferred embodiment according to thisinvention, the means for generating and emitting the referencedfrequency to thermally stable frequency discriminator 27 comprisesfrequency oscillator 26 having an output electrically coupled to aninput of frequency discriminator 27. It is apparent that such localoscillator can be any suitable oscillator or oscillating device known tothose skilled in the art. Likewise, individual oscillators 30-33 ofoscillator bank 29 can be combined to receive the output signal fromfrequency discriminator 27, in any suitable hardware manner known tothose skilled in the art. Summing device 35, mixers 41 and 45, andamplifiers 24, 37 and 54 can also be of any conventional hardwarecomponents known to those skilled in the art.

While in the foregoing specification this invention has been describedin relation to certain preferred embodiments thereof, and many detailshave been set forth for purpose of illustration, it will be apparent tothose skilled in the art that the invention is susceptible to additionalembodiments and that certain of the details described herein can bevaried considerably without departing from the basic principles of theinvention.

What is claimed is:
 1. A method for controlling frequency of amulti-channel transmitter, including the steps of:generating a referencefrequency and emitting the reference frequency to a first thermallystable frequency discriminator; emitting a first dc voltage from thefirst thermally stable frequency discriminator simultaneously to aplurality of individual oscillators; and combining a plurality ofindividual signals correspondingly emitted from the individualoscillators into an output signal having a multi-channel intermediatefrequency.
 2. In a method according to claim 1 wherein the multi-channelintermediate frequency is amplified and emitted as the output signal. 3.In a method according to claim 1 wherein the output signal is mixed witha control signal emitted at a control frequency from a local oscillatorto form a first mixed signal and the first mixed signal is amplified. 4.In a method according to claim 1 wherein the individual signals are eachat an intermediate frequency less than an output frequency of the outputsignal.
 5. In a method according to claim 1 wherein the individualsignals are different from each other and at least one fixed frequencyinterval is between adjacent said individual signals.
 6. In a methodaccording to claim 1 wherein the individual signals are amplified beforethe individual signals are modulated and combined.
 7. A method forcontrolling frequency of a multi-channel transmitter, including thesteps of:generating a reference frequency and emitting the referencefrequency to a first thermally stable frequency discriminator; emittinga first dc voltage from the first thermally stable frequencydiscriminator to a plurality of individual oscillators; combining aplurality of individual signals correspondingly emitted from theindividual oscillators into an output signal having a multi-channelintermediate frequency; and mixing the output signal with a controlsignal emitted at a control frequency from a local oscillator to form afirst mixed signal and amplifying the first mixed signal, and mixing asample of the control signal with a frequency reference signal generatedfrom a stable frequency source to form a second mixed signal, emittingthe second mixed signal to a second thermally stable frequencydiscriminator, and emitting a second dc voltage from the secondthermally stable frequency discriminator which is received by the localoscillator.
 8. In a method according to claim 7 wherein the sample ofthe control signal is taken from a directional coupler exposed to thecontrol signal.
 9. In a method according to claim 7 wherein the secondmixed signal is amplified.
 10. A frequency controller for amulti-channel transmitter, the frequency controller comprising:a firstthermally stable frequency discriminator, first means for generating andemitting a reference frequency to said first thermally stable frequencydiscriminator; and a plurality of individual oscillators combined tosimultaneously receive a discriminator output signal emitted from saidfirst thermally stable frequency discriminator, second means forcombining a plurality of individual signals emitted from saidoscillators into an output signal having a multi-channel intermediatefrequency.
 11. A frequency controller according to claim 10 furthercomprising third means for amplifying the multi-channel intermediatefrequency to form the output signal.
 12. A frequency controlleraccording to claim 10 further comprising: a local oscillator emitting acontrol signal at a control frequency, and fourth means for mixing saidcontrol signal with said output signal and emitting a first mixedsignal.
 13. A frequency controller according to claim 12 furthercomprising fifth means for amplifying said first mixed signal.
 14. Afrequency controller according to claim 10 wherein said individualsignals are each at an intermediate frequency less than an outputfrequency of said output signal.
 15. A frequency controller according toclaim 10 wherein said individual signals are different from each otherand at least one fixed frequency interval is between adjacent saidindividual signals.
 16. A frequency controller according to claim 10wherein said individual signals are amplified before said individualsignals are combined.
 17. A frequency controller for a multi-channeltransmitter, the frequency controller comprising:a first thermallystable frequency discriminator, first means for generating and emittinga reference frequency to said first thermally stable frequencydiscriminator; a plurality of individual oscillators combined to receivea discriminator output signal emitted from said first thermally stablefrequency discriminator, second means for combining a plurality ofindividual signals emitted from said oscillators into an output signalhaving a multi-channel intermediate frequency; and a local oscillatoremitting a control signal at a control frequency, fourth means formixing said control signal with said output signal and emitting a firstmixed signal, a second thermally stable frequency discriminator, astable frequency source generating a frequency reference signal, fifthmeans for mixing said control signal with said frequency referencesignal to form a second mixed signal and to emit said second signal tosaid second thermally stable frequency discriminator, and said localoscillator receiving a second voltage emitted from said second thermallystable frequency discriminator.
 18. A frequency controller according toclaim 17 further comprising a directional coupler exposed to saidcontrol signal.
 19. A frequency controller according to claim 17 furthercomprising sixth means for amplifying said second mixed signal.